Why are all the hymns in Calpurnia's church sung by lining?

HOW- it started in African American churches. the attendents that could read, would read off a line and the congegation 'sings' it back they do that thru the whole song.

WHY- not everyone was aloud to read back then (1700-1800's ish) it was one way for the people that could not read to sing praise...